Oecologia (2002) 131:137–144 DOI 10.1007/s00442-002-0874-z COMMUNITY ECOLOGY Andreas Floren · Alim Biun · K. Eduard Linsenmair Arboreal ants as key predators in tropical lowland rainforest trees Received: 24 September 2001 / Accepted: 2 January 2002 / Published online: 14 February 2002 © Springer-Verlag 2002 Abstract Ants numerically dominate the canopy fauna on the ground and lower vegetation (Hölldobler and of tropical lowland rain forests. They are considered to Wilson 1990), and fogging studies demonstrate that they be key predators but their effects in this regard have only also dominate in the canopy (Erwin 1983; Stork 1991; rarely been studied on non-myrmecophytes. A conspicu- Floren and Linsenmair 1997; Wagner 1997; Adis et al. ously low abundance of less mobile, mainly holometa- 1998). A conspicuously low abundance of less mobile bolous arthropods like Lepidoptera larvae corresponds arthropods in the trees, mainly larvae of holometabolous with ant dominance, while hemimetabolous highly taxa like Lepidoptera- or Coleoptera, which contribute mobile nymphs occur regularly and in large numbers in less than 1% to a maximum 3% of all arthropods, corre- the trees. This is in contrast to the temperate regions lates with ant dominance. In contrast, highly mobile where ants are mostly lacking on trees and holometabol- nymphs of hemimetabolous taxa provide, on average, ous larvae are frequent. In this study we experimentally 17% of individuals to a community (Floren and Linsenmair measured ant predation in the trees by offering caterpil- 1997,1998a; Stork 1991; Wagner 1997). This is in con- lars as baits. Fifty-four ant species were tested, of which trast to the temperate regions where tree communities 46 killed caterpillars and carried them away to their nests contain only few ants and numerous less mobile arthro- while only eight species ignored the offered larvae. pods (e.g. Horstmann 1976/1977; Southwood et al. 1982; Insecticidal knockdown fogging of ten trees after finish- Simandl 1993; Wagner 1996; A. Floren unpublished ing the prey experiments showed that on average 85% of data). These results suggest that the arboreal ants strongly ant individuals per tree were predacious. With the analy- influence the composition of the arthropod fauna in trees sis of another 69 foggings and meticulous observations of tropical lowland forests by exerting a high predation in many other trees this suggests that arboreal ants are pressure. This might also explain in part the extraordi- responsible for the low abundance of less mobile arthro- narily high dynamics in species composition in tree- pods in tropical lowland rain forest canopies. Ant preda- associated communities (expressed as fast species turnover) tion was significantly lower in a disturbed forest indicat- as found for example in daily re-fogging experiments of ing that human disturbance induces a change in the func- individual trees (Floren and Linsenmair 1997, 1999, tional interactions in these ecosystems. unpublished data; see also Stork 1991). Myrmecophilous and myrmecophytic plants make use of ants as protective Keywords Anthropogenic disturbance · Canopy · agents against herbivores by attracting ants with food Community structure · Ecosystem function · Fogging and/or shelter in specialized plant structures (Fiala and Maschwitz 1992; Fiala et al. 1994; Fonseca and Ganade Introduction 1996; Bronstein 1998; Heil et al. 2000, 2001; Oliveira et al. 1987; Letourneau and Dyer 1998). Because such trees Formicidae are considered to be the most abundant and are only a minority in primary forests, their special rela- most important predators in tropical lowland rain forests tionships provide us with no explanation of the general scarcity of less mobile arthropods in tropical lowland rain A. Floren (✉) · K.E. Linsenmair forest trees. Until now, however, no close attention has Department of Animal Ecology and Tropical Biology, been paid to the effects that ant predation pressure exert Biozentrum, Am Hubland. University Wuerzburg, 97074 Wuerzburg, Germany on the composition of arthropod communities in canopies e-mail: [email protected] of the large majority of trees that are neither myrmecophil- Tel.: +49-931-8884376, Fax: +49-931-8884352 ous nor myrmecophytes. A. Biun This is the first study to measure ant predation in Sabah Parks, Peti Surat 10626, 88806 Kota Kinabalu, Malaysia tropical lowland rain forest trees by offering caterpillars 138 as prey and by quantifying attack rates of various ant tree were tested. If ant individuals did not behave in a uniform species. By fogging ten trees after finishing the prey- way, the behaviour most often observed was used to categorize them. However, inconsistent results were only recorded in a few offering experiments we were able to assess the pro- cases. The caterpillars showed either no recognizable reaction or a portion of predacious to non-predacious ants in these clear defence and/or escape behaviour (strong beating of the body communities. in order to shake off the ants or spinning down from the branch or leaf on a thread of silk). In these experiments we assume that ant behaviour in response Materials and methods to the baits does not differ from those of ants which forage in a tree and accidentally meet a caterpillar. This assumption corre- sponds with the results of earlier studies (Goetzke 1993; Floren Study site and Linsenmair 1997; Berghoff 1998). In order to test whether ants that were attracted by tuna baits reacted differently compared Field work was done in January and February 2000 in the tropical to ants that met a larva accidentally, individual caterpillars were lowland rain forest of Kinabalu National Park, substation Poring put on leaves and observed until they were encountered by an ant. Hot Springs, in Sabah, Malyasia (6°2.75′N, 116°42.2′E). Details about the study area have been published elsewhere (Floren and Linsenmair 1997, 2000). Insecticidal fogging In order to determine relative proportions of predacious to non- Tree species chosen for the prey experiments predacious ants in individual communities, ten trees out of the same sample were fogged after the prey experiments were For carrying out the prey experiments we selected 45 trees of the finished. Because abundances of most ant species are represented lower canopy stratum, between 20 and 30 m high. Twenty-six trees in approximately correct proportions in the fogging samples, it is belonged to the species Aporusa lagenocarpa (Euphorbiaceae)*, possible to roughly assess – based on the comparison with the tuna the focal tree of our earlier studies (Floren and Linsenmair 1997, bait data – the ratio of predatory to non-predatory ant specimens in 2000), and ten were Depressa nervosa trees (Guttiferae). Further- a particular tree. Comprehensive information about the method of more, ants were tested that lived in the following trees: Barringtonia insecticidal fogging is given in Floren and Linsenmair (1997, scortechinii* (two trees), B. gitingensis (Lecythidaceae), Cryptoc- 2000, 2001) and Adis et al. (1998). Since we have no indication of arya sp. (Lauraceae), Payena lucida (Sapotaceae), Aporusa the existence of tree-specific ant species (Floren and Linsenmair maingayi (Euphorbiaceae)*, Dacryodes laxa (Burseraceae)*, 1997, 2000), the selection of the trees for fogging followed practical Palaquium rostratum (Sapotaceae)* and Ochanostachys amentacea criteria. Trees should be fully grown and should grow in similar (Olacaceae)*. Ten trees of the species marked with an asterisk habitats, as regards for example edaphic factors, degree of shading, were fogged after the prey experiments finished (see “Insecticidal etc. (Floren and Linsenmair 1998b). Tree structural parameters, fogging” below). such as the amount of dead wood, should be similar and tree cover by lianas and epiphytes should be low. No myrmecophilous nor myrmecophytic trees were investigated [however, a highly special- The prey species ized ant species of the genus Cladomyrma (Formicinae) which lived in a liana growing in a tree was tested in the prey experi- In most ecosystems, larvae of Lepidoptera are important herbi- ments]. The ten trees selected for fogging were four A. lagenocarpa, vores (e.g. Koptur 1984; Smiley 1987; Letourneau et al. 1993). two B. scortechinii, and the other trees given above. For our experiments we chose caterpillars of a Gelechiidae species, genus Onebala, which were found in leaf-rolls of Hibiscus shrubs. No other species of Lepidoptera larvae were found in Investigations in a disturbed forest greater abundance. Onebala specimens are not hairy and we have no indication that they are chemically defended (C. Schulze, Earlier investigations indicated that ant predation, and thus their personal communication). Caterpillars of two size classes were effects on arthropod communities, might differ between primary used in the experiments: (1) large individuals of 2–2.5 cm size, and disturbed forests. These investigations were carried out in and (2) small larvae of about 1 cm. three forests of different disturbance level. They were clear cut, used some years for crop planting, abandoned, and left to natural regeneration 5, 15, and 40 years ago. From each forest ten con- The prey-offering experiments specific trees were fogged. Details are given in Floren and Linsenmair (1998b) and Floren et al. (2001a). Three years ago, Because a number of ant individuals per species was needed for these forests were completely destroyed by a fire. the prey experiments, ants were attracted to the trees with pieces In order to test whether ant predation differed between the of tuna bait that were placed on leaves and most branches within primary and a disturbed forest we included a strongly disturbed reach of the stem. We have shown earlier that most arboreal ant forest in our study. It was around 500 m from the primary forest, species are generalist feeders that can be attracted with tuna baits. separated by private gardens and the entrance area of the National In fact, this method allows one to map the ant nests of a tree Park. The forest was used as pasture for water buffaloes.